When you take something without paying for it, that's theft. In the case of copyrighted material, it does not matter if you were going to buy it or not- you have it; you have received the value of the product without paying for it, again, that's theft. They were entitled to the purchase price of the material in question, you took it without paying, which means you stole the value of the purchase price.
If a game costs $20, and you download it without paying, then you have deprived the owner of $20; it doesn't matter if you were going to buy it or not, you still took it. Your argument is no different than someone shoplifting a candy bar and then saying: "It's not stealing because I wasn't going to buy it anyway"; one has nothing to do with the other. So yes, the Author's guild is absolutely correct in saying that every pirated copy represents lost revenue, because it does. Whether or not you were ever going to buy a copy of their book is completely irrelevant once you actually take it.
This is not a difficult concept to understand:
When you take something without paying for it, that's theft. In the case of copyrighted material, it does not matter if you were going to buy it or not- you have it; you have received the value of the product without paying for it, again, that's theft. They were entitled to the purchase price of the material in question, you took it without paying, which means you stole the value of the purchase price.
If a game costs $20, and you download it without paying, then you have deprived the owner of $20; it doesn't matter if you were going to buy it or not, you still took it. Your argument is no different than someone shoplifting a candy bar and then saying: "It's not stealing because I wasn't going to buy it anyway"; one has nothing to do with the other. So yes, the Author's guild is absolutely correct in saying that every pirated copy represents lost revenue, because it does. Whether or not you were ever going to buy a copy of their book is completely irrelevant once you actually take it.